Fixes #560 (#562)

* Fixes #560.

* Apply suggestions from code review

Co-authored-by: Cassandra Granade <chgranad@microsoft.com>

Co-authored-by: Cassandra Granade <chgranad@microsoft.com>

Author: msoeken

Numerics/src/FixedPoint/Convert.qs

@@ -0,0 +1,22 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+namespace Microsoft.Quantum.Convert {
+    open Microsoft.Quantum.Arrays;
+    open Microsoft.Quantum.Math;
+
+    /// # Summary
+    /// Returns the double value of a fixed-point approximation from of a `Bool` array.
+    ///
+    /// # Input
+    /// ## integerBits
+    /// Assumed number of integerBits (including the sign big)
+    /// ## bits
+    /// Bit-string representation of approximated number
+    internal function BoolArrayAsFixedPoint(integerBits : Int, bits : Bool[]) : Double {
+        let numBits = Length(bits);
+        let intPart = (Tail(bits) ? -(1 <<< (numBits - 1)) | 0) + BoolArrayAsInt(Most(bits));
+        return IntAsDouble(intPart) / PowD(2.0, IntAsDouble(numBits - integerBits));
+    }
+
+}

Numerics/src/FixedPoint/Measurement.qs

@@ -2,9 +2,10 @@
 // Licensed under the MIT License.
 
 namespace Microsoft.Quantum.Arithmetic {
+    open Microsoft.Quantum.Arrays;
+    open Microsoft.Quantum.Canon;
     open Microsoft.Quantum.Convert;
     open Microsoft.Quantum.Measurement;
-    open Microsoft.Quantum.Math;
 
     /// # Summary
     /// Measure a fixed-point number, returns its value as Double, and resets
@@ -15,18 +16,8 @@ namespace Microsoft.Quantum.Arithmetic {
     /// Fixed-point number to measure.
     operation MeasureFxP(fp : FixedPoint) : Double {
         let (p, xs) = fp!;
-        let n = Length(xs);
-        let sign = MResetZ(xs[n-1]) == One;
-        mutable keepAdding = sign;
-        mutable fpAsDouble = 0.;
-        for i in 0..n - 2 {
-            mutable currentRes = MResetZ(xs[i]) == (sign ? Zero | One);
-            if keepAdding {
-                set keepAdding = currentRes;
-                set currentRes = not currentRes;
-            }
-            set fpAsDouble = fpAsDouble * 0.5 + (currentRes == true ? 1. | 0.);
-        }
-        return (sign ? -1.0 | 1.0) * fpAsDouble * PowD(2.0, IntAsDouble(p-2));
+
+        let bits = ForEach(q => MResetZ(q) == One, xs);
+        return BoolArrayAsFixedPoint(p, bits);
     }
-}
+}

Numerics/tests/FixedPointTests.qs

@@ -3,6 +3,7 @@
 
 namespace Microsoft.Quantum.Numerics.ToffoliTests {
     open Microsoft.Quantum.Arrays;
+    open Microsoft.Quantum.Canon;
     open Microsoft.Quantum.Convert;
     open Microsoft.Quantum.Math;
     open Microsoft.Quantum.Intrinsic;
@@ -21,6 +22,33 @@ namespace Microsoft.Quantum.Numerics.ToffoliTests {
         }
     }
 
+    internal operation PrepareAsSignedAndMeasure(value : Int, fxp : FixedPoint) : Double {
+        ApplyXorInPlace(value, LittleEndian(Snd(fxp!)));
+        return MeasureFxP(fxp);
+    }
+
+    operation MeasureFxPTest() : Unit {
+        use qs = Qubit[4];
+        let qsFxP = FixedPoint(2, qs);
+
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b0000, qsFxP), 0.0);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b0001, qsFxP), 0.25);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b0010, qsFxP), 0.5);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b0011, qsFxP), 0.75);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b0100, qsFxP), 1.0);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b0101, qsFxP), 1.25);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b0110, qsFxP), 1.5);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b0111, qsFxP), 1.75);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b1000, qsFxP), -2.0);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b1001, qsFxP), -1.75);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b1010, qsFxP), -1.5);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b1011, qsFxP), -1.25);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b1100, qsFxP), -1.00);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b1101, qsFxP), -0.75);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b1110, qsFxP), -0.5);
+        NearEqualityFactD(PrepareAsSignedAndMeasure(0b1111, qsFxP), -0.25);
+    }
+
     operation CompareGreaterThanFxPTest() : Unit {
         for a in [1.2, 3.9, 3.14159, -0.6, -4.5, -3.1931, 0.0] {
             for b in [1.1, 3.95, 3.14259, -0.4, -4.6, -3.931, 0.1] {